Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (388 page)

   Platelets are markedly decreased in most acute cases. In chronic cases or when ITP develops insidiously, the decrease in platelet counts may be moderate to marginal.

   Peripheral blood smear (PBS) is normal, except for the decreased number of platelets that are frequently large (early, accelerated bone marrow release) and MPV is elevated. Platelet clumping must be excluded (pseudothrombocytopenia). Schistocytes are not present.

   Bone marrow is not indicated unless an underlying hematologic disease is suspected. Bone marrow examination should be performed in patients over the age of 60 to rule out a myelodysplastic syndrome. In ITP patients, there are an increased number of megakaryocytes, with a shift toward younger cells that do not seem to release platelets.

   Coagulation: All tests are normal.

   Serology to rule out SLE is mandatory in adult ITP. ANA may also be helpful.

   Platelet antibody detection and identification assays are available through reference laboratories. ELISA and flow cytometry methods are offered. However, because of the high frequency of false-positive and false-negative tests (antibodies are detected in only 60% of patients), serologic tests for platelet antibodies are not recommended.

   Microbiology: HIV and hepatitis C infections need to be ruled out in populations at risk.
Helicobacter pylori
testing may have pertinence since the elimination of certain strains may eradicate ITP.

   Blood group Rh (D) typing is necessary if anti-D Ig is being considered for therapy.

Suggested Readings

Gernsheimer T, James AH, Stasi R. How I treat thrombocytopenia in pregnancy.
Blood.
2013;121:38–47.

Liebman HA, Pullarkat V. Diagnosis and management of immune thrombocytopenia in the era of thrombopoietin mimetics.
Hematology Am Soc Hematol Educ Program.
2011;2011: 384–390.

DRUG-INDUCED THROMBOCYTOPENIA, IMMUNE

   Definition

Accelerated platelet destruction caused by drug-dependent, platelet-reactive antibodies.

Decreased platelet production by megakaryocytes may be a contributory factor.

   Who Should Be Suspected?

A bleeding patient with
isolated
thrombocytopenia and a history of medication known to result in drug-induced thrombocytopenia. The most common offenders are: quinine,
***
quinidine, heparin (discussed separately as HIT, see below), sulfa drugs, digoxin, GPIIb/IIIa antagonists, vancomycin, gold compounds, β-lactam antibiotics, valproic acid, levodopa, procainamide, and vaccines against measles– mumps–rubella. Criteria for diagnosis include a candidate drug administered shortly before the episode, especially if it was the only drug used, and exclusion of other causes for the acute thrombocytopenia.

   Laboratory Findings